regexp: fix latent stale flag bug after NFA prefix fast-forward

[le0pard]

In exec.go, when the NFA machine utilizes m.re.prefix to fast-forward the pos cursor, it updates the runes (r, r1) but fails to recalculate the flag context for the new position. This oversight causes the subsequent m.add() call to evaluate InstEmptyWidth using the stale boundary context from the pre-jump position.

Interestingly, this was implemented correctly in the OnePass engine (doOnePass explicitly calls flag = i.context(pos) after fast-forwarding), but was missed in the NFA match() loop.

Currently, this NFA bug is dormant because syntax.(*Prog).Prefix() halts at InstEmptyWidth, meaning any regex starting with a boundary assertion will have an empty prefix and bypass the fast-forward loop entirely. However, if the prefix extractor is ever optimized to identify literal prefixes after empty-width assertions, this stale flag will immediately cause false positives and negatives.

This commit adds the missing flag = i.context(pos) assignment to the NFA match loop. A test is also included which artificially injects a prefix into a boundary-prefixed Regexp to simulate a smarter compiler and prove the stale flag evaluation.

Found it while was busy with re2js replacing thread pool with a sparse array (because js env have no threads) - le0pard/re2js#48

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